While both polar and ferroelectric crystals have spontaneous polarizations, only the the ferroelectrics have a reversible polarization. There are 10 polar point groups: 1, 2, 3, 4, 6, m, mm2, 3m, 4mm and 6mm (in Hermann–Mauguin notation), and according to every reference I find, the ferroelectric point groups are some smaller subset of this set. However, everyone simply states that the point groups are "some subset" without giving a definitive list.
Are the ferroelectric point groups actually not known? Why can't we precisely state which ones are ferroelectric?